The use of analytical assays, including those used to determine the presence of drugs of abuse, has grown rapidly over the past decade. By 1993, the U.S. drug-testing market, alone, was estimated to be at least $500M. The drug-testing industry is poised for further growth as a result of new federal U.S. regulations that will significantly increase the number of workers subject to testing for drug and alcohol abuse.
At present, most drug testing involves sample collection followed by instrument-based "wet chemistry" laboratory analysis. However, the on-site, or "point of care" market has been growing rapidly over the past two years. Although no current figures are available, the market for non-instrumented immunoassay-based drugs of abuse test kits appears to be growing at the rate of 20-40% per year.
Currently, there are a number of single analyte immunoassay-based drugs of abuse diagnostic tests on the market. These include tests produced by Roche Diagnostic Systems, Hansen Hong Biomedical Co. Ltd., Drug Screening Systems, Editek, Inc., Hycor Biomedical, U.S. Drug Testing Inc., Thermedics Detection, Inc., and Fingerprint Biotek. Such devices generally work well for situations in which a specific drug is suspected. In many cases, however, such as in emergency room settings, it would be particularly desireable to have a rapid, self-performing assay for one or more drugs that may be present in a given patient.
A variety of assay kits have been described having the capability to perform diagnostic assays. For instance, a series of patents issued to Olson (U.S. Pat. Nos. 4,959,307; 4,963,468; 5,085,987; and 5,085,988) relate to an immunoseparating strip having a bibulous material, a nondiffusively bound first receptor, and a nondiffusively bound second receptor. In each embodiment, however, the method of using the device requires the first step of preparing a separate test solution containing the sample, antibody for the analyte, and a conjugate of analyte and a label. Once formed, the competitive reaction progresses to the desired extent in the solution phase. The solution is then transferred by the user to the contact portion of the analytical device, where it begins its flow along the path. (See, e.g., the U.S. Pat. No. '987, col. 13, lines 35-38 and col 20, lines 10-11.)
Others have disclosed the use of kits capable of performing two or more assays, including multi-analyte on-site formats. A kit available from Biosite ("Triage" brand), is said to allow the differential detection of the presence of several common drugs of abuse in a single urine or serum sample. See, for example, Buechler, et al, Clin. Chem. 38(9):1678-1684 (1992). At least one drawback of this device is the need to separately add sample to a region containing lyophilized reagents, where it is left for a period of time (e.g, 10 minutes), in order to allow the sample to reconstitute and equilibrate with the reagents.
This and other single and multi-analyte test kits currently on the market have several drawbacks. Present formats tend to be quite complex, with specific affinity constants playing a key role in the competitive binding reactions. Moreover, the formats can suffer from false results if the patient is on high doses of the analyte drug. Also, present formats typically require exact reagent concentrations (i.e., ratios of analyte to anti-analyte), which can be compromised if one member of the ligand-receptor pair begins to deteriorate.
Particularly troublesome are kits that rely on the use of an immobilized antibody or binding reagent, where the amount of this reagent needs to be rigidly controlled. The binding capacity of immobilized receptors can be highly dependent on the particular immobilization methods and conditions. This dependence causes the manufacture of such assays to be unpredictable and difficult to reproduce. Shelf life stability can be affected as well.
What is clearly needed is an analyte test kit that can be used for a number of different drugs of abuse, and particularly one that can be simultaneously used to perform a plurality of assays, in a manner that is easier to manufacture and simple and reliable in use.